The Increasing Demand for Telecommunications Services
Over the past few decades, the demand for access to information has increased dramatically. Although conventional wire and fiber landlines, cellular networks and geostationary satellite systems have continued to expand in an attempt to meet this relentless growth in demand, the existing capacity is still not sufficient to meet the burgeoning global appetite for telecommunications services.
Through technology advances and regulatory changes, mobile communication services were offered on a commercial basis and grew to meet city, regional, national and even international coverage needs through interconnection to public networks. As part of this evolution, wireless network standards have developed, on both a national and international basis, although there are still no truly international seamless wireless networks.
The decline in price of mobile services is one of the most important forces helping mobile communications reach broad-based markets and demonstrate rapid subscriber growth. The forces driving development of terrestrial wireless communications include advances in technology, declining prices and digital technology.
The resulting reductions in service and equipment cost attributable to the factors described above have allowed mobile communications to penetrate both business and consumer markets. The ultimate goal of wireless services is to provide two-way, ubiquitous and affordable communications services. It was only very recently, with the introduction of mobile satellite services, that this has been made possible. Indeed, mobile satellite services are the final step in the evolution of wireless communications service and are the only services which can provide this ultimate goal of ubiquitous wireless communication.
Terrestrial-Based Mobile Communications Services
Currently, there are five major types of public mobile communications services used throughout the world:
1. Cellular, which provides primarily two-way, interconnected voice service with mobile, transportable, and portable telephones and providing a platform for data transmission; PA1 2. Paging, which offers primarily one-way data transmission of numeric and alphanumeric messages; PA1 3. Private Radio/SMR, which supplies primarily two-way voice service to closed user groups, but may also provide interconnected and mobile data services. SMR is a subset of private radio where service is provided on a commercial basis to businesses by carriers instead of the businesses owning their own systems. PA1 4. Mobile Data, which provides networks for the exclusive transmission of mobile data; and PA1 5. Personal Communications Services (PCS), which uses microcell technology, includes a wide range of voice and data services, for example, one-way outgoing PCS services, called CT-2, licensed in several countries such as the U.K., Taiwan and the Netherlands.
The growth and evolution of mobile services show that subscribers migrate from basic limited services to more advanced services over time. The growth of terrestrial-based mobile services will increase the awareness and demand for enhanced mobile satellite services. Moreover, mobile satellite services will be able to provide service in areas that cannot be economically served using terrestrial networks.
Wireless Communications
As a result of the advances in technology, privatization, and decreasing prices on a world-wide basis, wireless communications have undergone a rapid increase in subscriber growth in the past several years. The result is that new enhanced wireless services tend to gain market acceptance more rapidly than did earlier wireless technologies. This phenomenon is attributable to the increasing functionality, value relative to price, and awareness among the population of each successive technology. Paging was introduced with only one-way, non-voice communications at a relatively high price. SMR provided two-way communications, but only within a closed user-group. Finally, cellular offered two-way interconnected voice with increasingly wide area coverage. The result of the rapid growth in wireless services worldwide builds an awareness and future demand for the benefits of advanced wireless communications.
Mobile Satellite Services
Mobile satellite services are uniquely positioned to complete the evolution of wireless services. These services offer ubiquitous coverage, interconnection with other networks and a variety of services.
Mobile satellites will be able to support both voice and data terminals, depending upon the particular need of the user. In general, however, voice service will be expensive relative to data, due to the greater infrastructure required for voice communications and the generally greater efficiency of data communications.
Several previous efforts to enhance world-wide communications capabilities are briefly described below. Robert R. Newton discloses a Multipurpose Satellite System in his U.S. Pat. No. 3,497,807. Newton describes a system in which "any point on Earth is always within the line of sight of some satellite and any satellite is always within the line of sight of an adjacent satellite in the same orbital plane." See Newton, Column 2, Lines 4-7.
U.S. Pat. No. 4,135,156 by Sanders et al., entitled Satellite Communications System Incorporating Ground Relay Station Through Which Messages Between Terminal Stations Are Routed, contains a description of a "satellite relay communications system" that "includes a ground relay station arranged so that each message from one subscriber to another is relayed by the satellite relay to the ground relay, processed by the ground relay and then transmitted to the second subscriber by way of the satellite relay." See Sanders et al., Abstract, Lines 1-6.
Paul S. Visher disclosed a Satellite Arrangement Providing Effective Use of the Geostationary Orbit in his U.S. Pat. No. 4,375,697. His patent recites a "satellite squadron or cluster formation" which "is disposed in a predetermined location in . . . geostationary orbit . . . " See Visher, Abstract, Lines 1-2.
In their U.S. Pat. No. 5,119,225, Michael Grant et al. explain their Multiple Access Communication System. The inventors describe a system that incorporates "a node spacecraft" in geostationary orbit that works in combination with "several user spacecraft" in low Earth orbit. See Grant et al., Abstract, Lines 1-3.
The references cited above disclose telecommunication systems that include satellites deployed in polar, Equatorial and inclined low Earth orbits (LEO). The systems provide for transmitting a message between two low-power fixed or mobile terminals on the ground through a store-and-forward network. The store-and-forward relay method takes advantage of the geometry of a system which allows the satellites to fly over different parts of the globe frequently. These LEO systems do not provide access to a satellite one hundred per cent of the time. In the most populated areas of the globe, a user may have to wait for many minutes until a satellite flies into view.
A burgeoning population of LEO communication satellites for commercial and military use is projected. Some observers estimate there will be 51 to 56 million users by the year 2002. To fill such a need for low-cost messaging and data communications available radio frequencies are required which may be packed into an already crowded very high frequency (VHF) and ultra high frequency (UHF) spectrum. Such spectrum already has many users.
Current satellite communication systems use different frequencies simultaneously to communicate between many satellites in the same constellation which are "visible" to a user on the Earth's surface. The term "visible" is an analogy which refers to the fact that radio energy at VHF and UHF frequencies travel essentially in line-of-sight directions. To prevent interference between a satellite to or from which a transmission is expected and other satellites in the same system requires that one satellite transmit and receive at a different frequency. Often the same frequency is used only for satellites in different orbital positions. This is so-called space division multiple access (SDMA). Interference among multiple satellites "visible" at the same point on the Earth may also be prevented by operating different signals at different polarization, or by use of orthogonal spread-spectrum codes. Time division multiple access (TDMA) is a further method of preventing satellites in a particular constellation from interfering with communications by others in the same constellation. The same frequency may be employed by different satellites with overlapping radio beams, but only at different times.
It would be a significant commercial advantage for a new system using a number of satellites to offer a user virtually immediate access to a satellite without interference. It would also be a commercial advantage, in some cases, to offer nearly instant, interference-free communication of the user's message to certain destinations. However, operation of numbers of new constellations of satellites presents a serious problem in that old constellations have been built and launched with little or no means of modifying their patterns of protection from communication interference.
The development of a system which would reduce or obviate interference between satellites of a new constellation and satellites of an already exiting constellation would constitute a major technological advance and would satisfy a long felt need in the satellite and telecommunications industries.